1. Field of the Invention
The present invention relates to an optical modulator and an optical transmitter used for optical transmission, and in particular relates to an optical modulator of a waveguide type which modulates light using an electro-optic effect, and to an optical transmitter which uses this.
2. Description of the Related Art
For example, an optical waveguide device which uses an electro-optic crystal such as lithium niobate (LiNbO3), lithium tantalate (LiTaO2) or the like, is manufactured by forming a metal film on a part of a crystal substrate and thermal diffusing, or by proton exchanging in benzoic acid after patterning, to thereby form an optical waveguide, and afterwards providing electrodes in the vicinity of the optical waveguide. As one optical waveguide device which uses an electro-optic crystal, there is known an optical modulator as shown for example in FIG. 28.
In general, the optical modulator, depending on the shape of the optical waveguide, is divided into a phase modulator as shown at the top of FIG. 28, and an intensity modulator as shown at the bottom of FIG. 28. In the case of the phase modulator, a signal electrode 131 is formed on one optical waveguide 102 which is formed on a substrate 101. Furthermore, in the case of the intensity modulator, the optical waveguide 102 comprises an input waveguide 121, a branching section 122, branching waveguides 123 and 124, a multiplexing section 125, and an output waveguide 126, and a coplanar electrode is formed with a signal electrode 131 provided on one branching waveguide 123, and a ground electrode 132 provided on the other branching waveguide 124.
In such an optical modulator, for example in the case where a Z-cut substrate 101 is used, the refractive index change due to an electric field in the Z-direction is used. Therefore, the electrode is arranged directly above the optical waveguide 102. More specifically, in the case of the intensity modulator at the bottom of FIG. 28, the signal electrode 131 and the ground electrode 132 are respectively patterned on the branching waveguides 123 and 124. At this time, in order to prevent the light propagated through the respective branching waveguides 123 and 124 being absorbed by the signal electrode 131 and the ground electrode 132, a buffer layer (not shown in the figure) is provided between the substrate 101 and the electrodes 131 and 132. For the buffer layer, an oxide silicon (SiO2) or the like of a thickness of 0.2 μm to 1 μm is used.
In the case of driving the above optical modulator at a high speed, the output terminal of the signal electrode 131 is made a traveling-wave electrode by grounding via a resistance (not shown in the figure), and a high frequency electric signal E such as a microwave is applied from an input terminal of the signal electrode 131. At this time, due to the electric field generated between the signal electrode 131 and the ground electrode 132, the refractive index of the optical waveguide 102 changes. Therefore, in the phase modulator at the top of FIG. 28, the phase of the light L propagated through the optical waveguide 102 is modulated in accordance with the electric signal E. Furthermore, in the intensity modulator at the bottom of FIG. 28, the refractive index of the branching waveguides 123 and 124 is changed for each, so that the phase difference of the light L propagated through each of these is changed, and an intensity modulated signal light L is output from the output waveguide 126.
In the optical modulator driven at high speed as described above, it is known that a wide band optical response characteristic is obtained by controlling the effective refractive index for the electric signal E by changing the cross-section shape of the signal electrode 131, and matching the propagation speed of the light L and the electric signal E. However, regarding the electric signal E propagated through the signal electrode 131, if the frequency thereof becomes high, the transmission losses increase. Therefore there is a problem in that the modulation band width is limited, so that high speed modulation becomes difficult.
As previous technology related to widening the band width of the optical modulator, for example as shown in FIG. 29, a configuration has been proposed where, of the interaction section for the light L and the electric signal E, the direction of the refractive index change is reversed by inverting the polarization direction in the remaining part 111 (surrounded by the dotted line in the figure) with respect to the polarization direction of the substrate 101 (direction of the crystal axis) of the part from the input side to a certain length (refer for example to Japanese Unexamined Patent Publication Nos. 2005-284129, 2005-221874, and 2006-47746). By means of this configuration, when modulation in the non-inversed region where the polarization direction is not changed is the forward direction, then in the polarization inversed region, modulation in the opposite direction occurs. That is to say, the polarization inversed part becomes an inverse modulated section, and the other part becomes a forward modulated section. As described above, since the loss of the electric signal E is great at high frequency, the intensity of the inverse modulation in the polarization inversed region is great at low frequency, and is small at high frequency. As a result, in the overall optical modulator, the modulation at low frequency is suppressed, so that the high frequency dependency is reduced, that is, the modulation bandwidth becomes wide.
Furthermore, as another conventional technology for improving the response characteristics of the optical modulator or the like, a configuration is also proposed where the electrode width of the signal electrode and the ground electrode is changed along the light propagation direction, to thereby prevent resonance of an acoustic wave (for example surface acoustic wave) which is produced when a modulating signal of a high pulse shape is applied between the electrodes, so that occurrence of ripple is suppressed, (refer for example to Japanese Unexamined Patent Publication No. 2000-275589).
However, in the conventional technology which gives a wider band width by using the above inverse modulation, since the modulation component of the high frequency band in the polarization inversed region (inverse modulation section) has not become small enough, then in the high frequency band, a certain amount of inverse modulation occurs. Therefore there is a problem in that the amount of improvement in bandwidth is limited.
Furthermore, in the conventional technology for improving the response characteristics by changing the electrode width of the signal electrode and the ground electrode along the light propagation direction, the influence on the light due to the resonance of the generated acoustic wave attributable to the piezoelectricity of the substrate can be reduced, but an increase in the propagation loss of the electrical signal in the high frequency as mentioned above cannot be effectively suppressed. Therefore there is the problem that it is difficult to realize a wider band width.